1. Field of the Invention
This invention relates to a fluidized bed flight apparatus. More particularly, the invention relates to a flight apparatus for an indexing fluidized bed which prevents the fluidized particles from becoming trapped under or passing under the flight.
2. Description of the Prior Art
Conventional fluidized bed apparatus are well known in the art. Such apparatus typically have a variety of uses such as dryers, reactors, surface coating devices, and particle size classifiers. The design of the apparatus is dependent upon the use to which the apparatus is put.
Typically, a fluidizing medium is flowed upward through a bed of solid particles. A relationship known to those skilled in the art and based on the size, shape, and density of the particles is utilized to establish a range of flow rates for the fluidizing medium so that the particles become fluidized, i.e., they are buoyed in the fluidizing medium in a fashion which allows the mass of solid particles to behave as if it were a fluid. The mass of fluidized particles assumes the shape of the vessel in which it is confined. Although the particles may still touch each other while fluidized, they are buoyantly suspended in the fluidizing medium so that lateral friction forces are reduced. This buoyancy does not result soley from the difference between the density of the particles and the density of the fluidizing medium. Rather, it results from the upward force exerted by the upwardly flowing fluidizing medium.
Particle shape affects the magnitude of the buoyant force exerted on the particle. At a given fluidizing medium velocity, the force exerted on the particle is proportional to the area of the particle upon which the flow impinges. The weight of the particle also affects the ability of the fluidizing medium to lift the particle. Indeed, the velocity of the fluidizing medium can be adjusted so that particles are lifted by, fall through, or are held statically in the fluidizing medium. The quantity of particles in the fluidizing medium can also be adjusted.
Designs of fluid bed apparatus are as varied as are the uses to which the apparatus are put. Fluidizing medium is flowed upward, typically through a distributor. The distributor is designed to ensure even distribution of the fluidizing medium while preventing solid particles from escaping from the bed. If it is desired to separate the fluidizing medium from the solid particles before the medium flows out of the vessel, there must be a way to retain particles in the vessel. Therefore, either a disengaging space or a mechanism for separating solid particles from the fluidizing medium must be provided. Examples of such mechanisms include cyclones and grids. Divers devices for introducing and removing solid particles from the fluidized mass are also known in the art.
Although the confining vessel of many fluidized beds is a vertical cylinder, other designs are utilized. For example, an indexing fluidized bed apparatus often is used when the duration of the contact between the solid particles and the fluidizing medium must be strictly controlled. An indexing fluidized bed apparatus comprises a device which ensures that solid particles are moved through the fluidizing medium within a predetermined period. Typically, such a device limits the length of the contact between particles and fluidizing medium by moving a confining vessel and physically urging particles through the fluidizing medium.
A typical indexing apparatus comprises flights on at least one endless device, such as a belt or roller chain, which is horizontally-disposed within a compartment having a grid as its bottom or floor. A plurality of such endless, or continuous, devices would be essentially parallel, equidistant from the grid, and spaced across the width of the compartment. Flights, which are rigid or semi-rigid, typically planar, partitions disposed from the continuous device, extend between side walls of the compartment and are moved through the compartment by moving the continuous device, as illustrated in FIG. 1. The grid distributes the fluidizing medium while preventing escape of particles. Thus, a confining vessel is formed by the grid, leading and trailing adjacent flights, the side walls, and a top which retains particles, and is moved through the fluidizing medium by the motion of the flights. The linear velocity of the device and the spacing of flights are adjusted to ensure that particles are exposed to fluidizing medium only for a predetermined period. If desired, the grid may be subdivided into zones in the direction the device travels, so particles can be sequentially exposed to a plurality of fluidizing medium.
Various designs for attaching flights to the continuous device are known in the art. For example, the top of the flights may be attached to the device, with the flights extending from the device to the grid, as shown in FIG. 1. Alternatively, the continuous device may be disposed on the side of the flight, so that the flight extends both above and below the device. Other relationships between the flights and the continuous device are known to practitioners.
If the continuous device is at the top of the flight, it may serve to keep the particles in the fluidized bed while allowing fluidizing medium to flow around or through it. More typically, however, the side walls of the compartment are not parallel, but are spaced further apart at the top of the bed than they are at the grid, as illustrated in FIGS. 1a and 1b. Such a design serves to retain particles in the confining vessel. As those skilled in the art recognize, the vertical velocity of the fluidizing medium is not constant, but decreases as the side walls diverge, and the relationship between the fluidized particles and the fluidizing gas can be manipulated in accordance with well-known particles to ensure that particles remain fluidized without escaping over the flights.
Inevitably, there will be spaces between the flight and the walls and bottom of the compartment. Such spaces are required to allow for manufacturing tolerances, mechanical clearances, and the like. These gaps typically are minimized, with the flight being able to urge particles out of the fluidizing medium without deleteriously effecting the particle of damaging either the particle or the flight. It is clear that particles which will overreact, burn, break, or otherwise be damaged when overexposed to fluidizing medium must be urged out of the fluidizing medium in a timely fashion. Therefore, the acceptable gap is related to the size of the particles and the necessity of removing the particles from the fluidizing bed undamaged and before the particles are over-exposed to the fluidizing medium. Similarly, particles cannot be allowed to enter the gap if the particle (or indeed the flight itself) will be damaged if the particle is caught in the gap.
Because the confining vessel is moved through the fluidizing medium by moving the flights, particles typically are trapped only by the trailing flight. Various methods have been proposed for ensuring that fluidized particles cannot pass under the trailing flight or become trapped in the gap between the flight and the grid. One such proposal calls for the attachment of strips of flexible material such as rubber or Teflon.RTM. to the flight so that the flexible material contacts the grid. However, this technique is not satisfactory because particles because trapped between the flexible material and the stationary grid. The particles are therefore scraped along the grid as the flight moves, thereby damaging the grid, the flight, or the particles. Others have proposed that a more rigid yet flexible material such as thin stainless steel be attached to the flight to contact the stationary grid. However, either this material or the grid will be abraded by the contact. The abraded material will become fluidized and mix with the solid particles. Such added material is often unacceptable, for example, where the fluidized material is a food product or component thereof.
It is an object of this invention to provide apparatus for preventing particles fluidized in an indexing fluidized bed from passing under the trailing flight or becoming trapped between the flight and the bottom of the fluidizing compartment.
It is a further object of this invention to provide a flight for an indexing fluidizing bed apparatus which prevents fluidized particles from passing under the trailing flight or becoming trapped between the flight and the bottom of the fluidizing compartment.
It is another object of this invention to provide a flight for indexing fluidizing bed apparatus which directs the flow of fluidizing medium to prevent fluidizing particles from passing under the flight or becoming trapped between the flight and the grid.
It is yet another object of this invention to provide additional agitation in an indexing fluidized bed to ensure thorough mixing or aid in preventing agglomeration.
It is still further an object of this invention to provide a method for preventing fluidized particles from passing under a flight or becoming trapped between the flight and the grid in an indexing fluidized bed.